Injection moulding feed hopper

The earliest injection moulding machines were of the plunger type as illustrated in Fig. 4.30 and there are still many of these machines in use today. A predetermined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heaters. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel as shown. 

A typical temperature sequence for injection molding DMC would be feed hopper and feed zone - ambient temperature metering section 50-60°C nozzle 80-90°C mould temperature 135-185°C for polyester DMC or 160-22°C for epoxy DMC injection pressure 80-160 MPa. Cure time is generally of the order of 10-20 s per millimetre of wall thickness. Very little finishing of moldings is necessary. 

All four forms can be processed using conventional compression moulding and transfer moulding techniques. These materials can also be handled satisfactorily on reciprocating screw injection moulding machines. However, it is necessary to modify the feed hopper in order to injection mould successfully the rope and putty types. 

Pre-drying Absolutely necessary Circulating air dryer 5-8 h, 110-120 °C dehumidifier 3-5 h, 120-140 °C. PAR only absorbs little humidity (max. 0.5%), but it absorbs it very quickly for processing the hiunidity should not exceed 0.02% more humidity leads to a reduction of the mould fill behaviour and leads to part surface waviness. To avoid the absorption of humidity inside the feed hopper of the injection moulding machine the hopper should be heated or a dry feeder used. 

Molding compound is transferred in the cold state by pressure from the material hopper into the main injection chamber. Here it can be preheated before injection into the heated mould tool. Injection, through a special nozzle, can be either by ram or screw pressure. If screw feed is used, the screw must be of the type designed to process thermosets as opposed to thermoplastics. 

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